An experimental study was conducted to determine the morphology of oil/water mixtures produced into a high water-cut well. This information is important for determining the feasibility of downhole oil/water separation.

Core-flow and microflow experiments were performed to simulate the two-phase flow process in the near-wellbore region of a high water-cut well. In the core-flow experiments the morphology of oil/water mixtures leaving different types of porous media was observed and the droplet-size distribution was determined for different fluid properties and flow parameters. In the microflow experiments the flow regimes and breakup mechanisms of the oil phase in the pore space were visualized and observed.

The experimental results show that the oil/water mixture emerging from the porous medium depends strongly on its wettability. When the porous medium is oil-wet, the produced oil is mainly in the form of large drops that are formed at the porous end face. From water-wet porous media, small oil droplets are produced at high flow rates; their sizes are typically of the pore size's order and are formed inside the porous medium. A correlation between the droplet size and relevant fluid, and flow and porous-medium parameters has been developed, allowing for the assessment of circumstances in which downhole separation can be problematic. The experimental results have also shown that in water-wet porous media a transition from capillary-dominated to dispersed flow takes place in the capillary number range investigated (10-5<Nc<10-3).

Introduction

Large amounts of water are produced together with oil in mature oil fields. The water cut can, in some cases, rise to almost 100 vol% before the wells are abandoned, depending on the well's economics. A new technology that could extend the economic lifetime of high water-cut oil wells is downhole separation. This technology comprises the partial separation of oil and water by a hydrocyclone mounted in the wellbore near the producing interval of the oil field. One factor determining the potential of this technology is the separation efficiency of the hydrocyclone, which depends strongly on the morphology of the oil/water mixture flowing into it.

An experimental investigation has been carried out, aiming to establish the morphology of an oil/water mixture as present in the near-wellbore formation and produced into a well at high water-cut conditions.

Two types of experiments were conducted: core-flow and microflow experiments. In the core-flow experiments the cocurrent flow of oil and water, occurring in the near-wellbore region, was simulated by injecting both phases simultaneously into different types of porous media. The type of oil/water mixture leaving the porous medium was observed and characterized, and the droplet-size distributions of any oil-in-water dispersions produced were measured by means of laser diffraction. The flow, fluid, and porous-medium parameters were varied in the experiments, and the water pressure was measured in the porous medium as a function of distance from the inflow face.

In the microflow experiments, the flow regimes and breakup mechanisms occurring in the porous medium of a two-dimensional (2D) etched-glass model were observed during cocurrent flow of oil and water. The parameters varied were flow rate, oil/water ratio, oil viscosity, interfacial tension, and wettability. The pressure drop over the model was also measured.

Theory

The vertical inflow profile of a high water cut is characterized by a top layer of oil, a bottom layer of water, and an intermediate layer in which both phases are present in the well (see Fig. 1). The presence of both phases in the intermediate layer is caused by static capillary forces and by a macroscopic instability at the flowing oil/water interface.1 The interval over which both phases are present is given by the capillary transition height hcap, which is the height of a column of the denser phase that can be supported by capillary forces. It is given by

Equation 1

where ??=density difference between oil and water phases, ? and s=contact-angle and interfacial tension of the oil-water interface, respectively, and rp=pore radius. An approximation often used for rp is v(8k/f), where k=permeability and f=porosity. In a water-wet sandstone formation with a permeability of 1 Darcy, hcap has a value of about 7 m.

In the pore space very close to a high water-cut well, the capillary number Nc attains values typically between 10-3 and 10-5, which can cause dispersion of the nonwetting phase.2,3 Several researchers2,3 have investigated the two-phase flow regimes occurring at the pore scale in porous media as a function of the capillary number. They have found that in completely water-wet porous media the connected oil paths, which prevail at low capillary numbers, start to break up when the capillary number becomes larger than about 10-5, provided the oil saturation is not too high. Upon increasing the capillary number from 10-5 to 10-3, the flowing oil entities change from large ganglia, with a typical length of 10 pore chambers, to so-called singlets, filling only one pore chamber. This result is consistent with the observation that during residual oil mobilization from water-wet sandstone, the size of oil entities that can be mobilized at Nc=10-3 is typical of the order of a pore body, which explains the fact that at this capillary number, the porous medium is nearly desaturated. In intermediate or oil-wet porous media, the flow regimes occurring are very distinct from those observed in completely water-wet porous media. This is caused by the high value of the adhesion force between the oil and the oil-wet porous medium, compared to the water-wet case.

When the porous medium is completely water-wet, a layer of water will always be present between the pore wall and the oleic phase. The phenomenon of oil breakup in the porous medium under water-wet conditions is caused by two different mechanisms: snap-off and viscous breakup.